This invention relates to a system and method for acoustically detecting cutting tool breakage and rejecting background noise spikes that could cause false alarms, and more particularly, for acoustically detecting breakage of carbide groove tools used in cutting grooves during lathing operations.
A Machine Tool Monitor to detect broken tools and for part probing is described in commonly assigned U.S. Pat. Nos. 4,636,779, 4,636,780 and 4,707,687 and in published technical papers. A single sensor such as an accelerometer is mounted on the machine tool in a location with good coupling to vibrations generated at the tool-workpiece interface. The system is programmed to recognize signal patterns resulting from tool breakage. These patterns are typically abrupt, substantial increases or decreases in the cutting noise mean signal level that persist for a given confirmation period and are caused by sudden changes in cutting conditions resulting from critical geometry changes in the cutting edge. Another pattern is a gradual decrease in the cutting noise signal level due to a series of small breaks or other gradually occurring breakage.
A lathe tool break detection system has been developed that consists of a high frequency accelerometer to convert metal-cutting operation vibrations to broadband electrical signals, analog signal processing circuitry to amplify a selected band of signal frequencies and detect the energy in that band, and digital time-domain pattern-recognition logic to detect tool break signatures and reject normal cutting operation artifacts in the processed vibration signal. Pattern recognition logic has been developed which detects tool break events occurring in ceramic materials, for instance, commonly assigned U.S. Pat. Nos. 4,636,779 and 4,636,780. This tool break detection system has been successfully used for the detection of tool breaks in metal-cutting lathe operations in which the tools are various ceramic materials of round shape, and the workpiece materials are tough aerospace alloys such as Inconel. Tests with carbide tools cutting Inconel and other metals have shown the high frequency acoustic signal produced by a tool break event is often a dense high amplitude spiky noise.
It has been found, however, that the acoustical patterns recognized in ceramic and carbide cutting tool breaks are different from those of groove tool breaks, even if the groove tool is made from carbide. Thus, the logic used in the pattern recognition of the described ceramic tool break events is not suited for the detection of carbide groove tool breaks, and when used for this application, it produces frequent false alarms and even fails to detect some tool break events. This unsatisfactory performance is readily explained by the characteristics of both groove tool break signatures and groove tool normal cutting vibration signals, which differ materially from the corresponding signatures and signals when round ceramic tools or non-groove cutting carbide tools are used. Hence, there is a need for identifying combinations of signal properties which can be used to separate groove tool break signatures from normal groove tool cutting vibration signals, and to devise pattern recognition logic which can perform this function reliably.
A different set of characteristics apply to groove cutting or cutoff operations. The nature of grooving and cutoff operations results in the use of an insert that is inherently weak. Obviously, it must be no wider than the groove it is to cut. Furthermore, since the insert is surrounded on three sides by the workpiece, it must be relieved top to bottom on at least two sides as well as front to back. Also, the insert generally does not have a great deal of support, because the toolholder itself must be narrow and relieved. Due to the delicate nature of the grooving and cutoff operation, turning rates of the lathe holding the workpiece must be slower and feed rates must be lighter than in other types of turning operations to avoid insert breakage.
Carbide groove tools are used extensively in the manufacture of aircraft engine parts. Because of their peculiar geometry and the slow cutting speeds at which they operate, the normal cutting vibration signals and tool break signatures produced in groove tool operations are both very different from their respective counterparts in most other machining operations. Consequently, vibration signal-based tool break detection techniques developed for other machining operations work poorly when applied without modification to groove tool operations. This invention exploits the normal cutting vibration signal and tool break signature characteristics of groove tool operations to produce a reliable groove tool break detector.
Normal groove tool cutting vibration signals, after analog signal processing, differ from corresponding round ceramic tool vibration signals in several major ways. The mean signal level tends to be less than one per cent of the normal ceramic tool signal mean level, a result that is due primarily to the much lower surface speed used in groove tool operations. This low mean signal level is, in turn, one of the primary reasons why the groove tool vibration signal has many more short noise spikes with far higher spike peak-to-mean signal level ratios. Such noise spikes are produced by chip dynamics and other normal metal-cutting phenomena in both cases. In the ceramic tool case they are obscured and often completely hidden by the high mean signal level, but in the groove tool case they are not. Groove tool vibration signals also tend to drop out abruptly, and essentially completely, shortly after the beginning of a cut. This is apparently related to the peculiar geometry of groove tools and the slots they are often used to cut in workpieces.
The fracture of tools made of carbide or other metal materials generates vibrational energy signals having short, high amplitude spikes. In ceramic tool operations, however, these tool break acoustic emission spikes are often obscured by the mean cutting noise. In groove tool operations, the tool break spikes are readily seen because their amplitudes are many times the low background mean cutting noise. The effect of a ceramic tool fracture on the cutting vibration signal mean level is most often an abrupt and sustained mean level decrease, but several other tool break signature types can occur. The abrupt and sustained mean level drop is even more dominant in groove tool break events. Because of the particular characteristics of the acoustical pattern generated by groove tool break events, a method and system specifically designed to detect and recognize their acoustical patterns is necessary.